Current instrument panels, such as are used by pilots in the cockpit of an aircraft, for example, normally provide indicators which are often illuminated by incandenscent lights which are bulky and must often be replaced. In an effort to avoid the use of incandescent bulbs, panel members have been suggested which are illuminated by electroluminescent (EL) lamps. The EL lamps are encased between two plexiglass sheets which are bonded together substantially around the edges of the EL lamps themselves. However, such structures are subject to mechanical failure due to cracking or other breakage problems as well as to bond line failures. Failure at the mounting hole locations for the panel member, for example, has often required the use of crush-proof grommets. Moreover, oversized or elongated mounting holes are normally required in order to accommodate the high coefficient of expansion of plexiglass material relative to that of the EL lamps themselves and to the mounting rack to which the panel members are attached, which mounting rack is often made of aluminum.
In order to provide adequate mechanical strength to avoid such problems, the plexiglass sheets are usually made relatively thick so as to be able to withstand the stresses which give rise to breakage problems. Moreover, under severe environmental conditions (i.e., high temperature, high vibrations and the like) the bonding maerial used to bond the plexiglass sheets together at the edges of the lamp elements tends to lose its effectiveness and the plexiglass sheets begin to separate so that the mechanical integriity of the overall panel member is lost.
It is desirable, therefore, to provide panel members which can use EL lamps and which can maintain its structural integrity over time, while at the same time reducing the panel thickness usually required and, hence, the weight and cost of the overall panel structure. Further, it is desirable that such panele members be capable of being molded into various contours as may be desired in a particular application, without subsequent warpage during use.